Method for producing silver nano-particles and silver nano-particles

ABSTRACT

The present invention provides a silver nano-particle production method which is safe and simple also in terms of scaled-up industrial-level production, in a so-called thermal decomposition method in which a silver-amine complex compound is thermally decomposed to form silver nano-particles. A method for producing silver nano-particles comprising: mixing an aliphatic hydrocarbon amine and a silver compound in the presence of an alcohol solvent having 3 or more carbon atoms to form a complex compound comprising the silver compound and the amine; and thermally decomposing the complex compound by heating to form silver nano-particles.

TECHNICAL FIELD

The present invention relates to a method for producing silvernano-particles and silver nano-particles. The present invention isapplied also to a method for producing metal nano-particles containing ametal other than silver and metal nano-particles.

BACKGROUND ART

Silver nano-particles can be sintered even at a low temperature.Utilizing this property, a silver coating composition containing silvernano-particles is used to form electrodes or conductive circuit patternson a substrate in production of various electronic devices. Silvernano-particles are usually dispersed in an organic solvent. Silvernano-particles have an average primary particle diameter of aboutseveral nanometers to about several tens of nanometers, and theirsurfaces are usually coated with an organic stabilizer (protectiveagent). When the substrate is a plastic film or sheet, silvernano-particles need to be sintered at a low temperature (e.g., at 200°C. or less) less than a heat resistant temperature of the plasticsubstrate.

Particularly, attempts have been recently made to form fine metal lines(e.g., silver lines) not only on heat-resistant polyimide substratesthat are already in use as substrates for flexible printed circuitboards but also on substrates made of various plastics, such as PET(polyethylene terephthalate) and polypropylene, that have lower heatresistance than polyimide but can be easily processed and are cheap.When plastic substrates having low heat resistance are used, metalnano-particles (e.g., silver nano-particles) need to be sintered at alower temperature.

Silver nano-particles have an average primary particle diameter of aboutseveral nanometers to about several tens of nanometers, and are morelikely to agglomerate than micron (μm)-size particles. Therefore, thereduction reaction of a silver compound (thermal decomposition reactionof a silver oxalate complex compound) is performed in the presence of anorganic stabilizer (protective agent such as an aliphatic amine or analiphatic carboxylic acid) so that the surfaces of resulting silvernano-particles are coated with the organic stabilizer.

Meanwhile, silver nano-particles are used in a silver coatingcomposition (silver ink or silver paste) in which the particles arecontained in an organic solvent. In order to development conductivity,an organic stabilizer coating the silver nano-particles needs to beremoved during calcining performed after application of the silvercoating composition onto a substrate to sinter the silver particles.When the temperature of the calcining is low, the organic stabilizer ispoorly removed. When the silver particles are not sufficiently sintered,a low resistance value cannot be achieved. That is, the organicstabilizer present on the surfaces of the silver nano-particlescontributes to the stabilization of the silver nano-particles, but onthe other hand, interferes with the sintering of the silvernano-particles (especially, sintering by low-temperature calcining).

The use of an aliphatic amine compound and/or an aliphatic carboxylicacid compound each having a relatively long chain (e.g., 8 or morecarbon atoms) as an organic stabilizer makes it easy to stabilize silvernano-particles because it is easy to ensure space between the silvernano-particles. On the other hand, the long-chain aliphatic aminecompound and/or the long-chain aliphatic carboxylic acid compound are/ispoorly removed when the temperature of calcining is low.

As described above, the relationship between the stabilization of silvernano-particles and the development of a low resistance value bylow-temperature calcining is a trade-off.

For example, JP-A-2008-214695 discloses a method for producing silverultrafine particles, comprising reacting silver oxalate and oleylamineto form a complex compound containing at least silver, oleylamine and anoxalate ion; and thermally decomposing the formed complex compound toform silver ultrafine particles (claim 1). Further, JP-A-2008-214695discloses that in the above method, a saturated aliphatic amine having 1to 18 carbon atoms in total is reacted in addition to the silver oxalateand the oleylamine (claims 2 and 3), so that a complex compound can beeasily formed, the time required to produce silver ultrafine particlescan be reduced, and the silver ultrafine particles protected by theseamines can be formed in higher yield (paragraph [0011]). Further, thesame document discloses that a solvent such as methanol, ethanol, orwater may be added when forming a complex compound (paragraph [0017]).

JP-A-2010-265543 discloses a method for producing coated silverultrafine particles, comprising the first step of mixing a silvercompound that is decomposed by heating to generate metallic silver, amid- to short-chain alkylamine having a boiling point of 100° C. to 250°C., and a mid- to short-chain alkyldiamine having a boiling point of100° C. to 250° C. to prepare a complex compound containing the silvercompound, the alkylamine and the alkyldiamine; and the second step ofthermally decomposing the complex compound (claim 3, paragraphs [0061]and [0062]). Further, the same document discloses that there are norestrictions on using a reaction solvent such as methanol, or water toform a complex compound (paragraph [0068]).

CITATION LIST Patent Documents

-   Patent Document 1: JP-A-2008-214695-   Patent Document 2: JP-A-2010-265543

SUMMARY OF INVENTION Technical Problem

JP-A-2008-214695 discloses that a solvent such as methanol, ethanol, orwater may be added to form a complex compound. However, the use ofmethanol or ethanol cannot increase the temperature of a reaction forthermally decomposing the complex compound, and therefore the complexcompound is not sufficiently thermally decomposed, which has an adverseeffect on forming silver particles. The use of water causes difficultyin completely removing water after forming silver particles.

JP-A-2010-265543 discloses that there are no restrictions on using areaction solvent such as methanol or water to form a complex compound.However, the use of methanol cannot increase the temperature of areaction for thermally decomposing the complex compound, and thereforethe complex compound is not sufficiently thermally decomposed, which hasan adverse effect on forming silver particles. The use of water causesdifficulty in completely removing water after forming silver particles.

On the other hand, when forming a complex compound of a silver compoundsuch as silver oxalate and an amine is conducted in the absence ofsolvent, it is very difficult to perform stirring because the silvercompound is in powder form. In addition, the reaction between the silvercompound and the amine is accompanied by heat generation, and thereforethere is a high safety risk in terms of scaled-up industrial-levelproduction.

As described above, regarding a so-called thermal decomposition methodin which a complex compound of a silver compound such as silver oxalateand an amine is thermally decomposed to form silver nano-particles, nosilver nano-particle production method has been so far developed, whichis safe also in terms of scaled-up industrial-level production, andwhich is capable of producing silver nano-particles that can be sinteredat a low temperature.

It is therefore an object of the present invention to provide a silvernano-particle production method which is safe and simple also in termsof scaled-up industrial-level production, in a so-called thermaldecomposition method in which a silver-amine complex compound isthermally decomposed to form silver nano-particles. It is also an objectof the present invention to provide a silver nano-particle productionmethod which is safe and simple also in terms of scaled-upindustrial-level production, and which is capable of producing silvernano-particles that can be sintered at a low temperature, in a so-calledthermal decomposition method in which a silver-amine complex compound isthermally decomposed to form silver nano-particles.

Solution to Problem

The present invention includes the following aspects.

(1) A method for producing silver nano-particles comprising:

mixing an aliphatic hydrocarbon amine and a silver compound in thepresence of an alcohol solvent having 3 or more carbon atoms to form acomplex compound comprising the silver compound and the amine; and

thermally decomposing the complex compound by heating to form silvernano-particles.

(2) The method for producing silver nano-particles according to theabove (1), wherein the silver compound is silver oxalate.

(3) The method for producing silver nano-particles according to theabove (1) or (2), wherein the aliphatic hydrocarbon amine comprises analiphatic hydrocarbon monoamine (A) comprising an aliphatic hydrocarbongroup and one amino group, said aliphatic hydrocarbon group having 6 ormore carbon atoms in total, and

further comprises at least one of an aliphatic hydrocarbon monoamine (B)comprising an aliphatic hydrocarbon group and one amino group, saidaliphatic hydrocarbon group having 5 or less carbon atoms in total; andan aliphatic hydrocarbon diamine (C) comprising an aliphatic hydrocarbongroup and two amino groups, said aliphatic hydrocarbon group having 8 orless carbon atoms in total.

(4) The method for producing silver nano-particles according to theabove (3), wherein the aliphatic hydrocarbon monoamine (A) is analkylmonoamine having 6 or more and 12 or less carbon atoms.

(5) The method for producing silver nano-particles according to theabove (3) or (4), wherein the aliphatic hydrocarbon monoamine (B) is analkylmonoamine having 2 or more and 5 or less carbon atoms.

(6) The method for producing silver nano-particles according to any oneof the above (3) to (5), wherein the aliphatic hydrocarbon diamine (C)is an alkylenediamine in which one of the two amino groups is a primaryamino group, and the other is a tertiary amino group.

(7) The method for producing silver nano-particles according to any oneof the above (1) to (6), wherein the alcohol solvent is selected fromthe group consisting of butanols and hexanols.

(8) The method for producing silver nano-particles according to any oneof the above (1) to (7), wherein the alcohol solvent is used in anamount of 120 parts by weight or more per 100 parts by weight of thesilver compound.

(9) The method for producing silver nano-particles according to any oneof the above (1) to (8), wherein the aliphatic hydrocarbon amine is usedin a total amount of 1 to 50 moles per 1 mole of silver atoms in thesilver compound.

A molecule of silver oxalate contains two silver atoms. The method forproducing silver nano-particles according to any one of the above (1) to(8), wherein when the silver compound is silver oxalate, the aliphatichydrocarbon amine is used in a total amount of 2 to 100 moles per 1 moleof silver oxalate.

-   -   The method for producing silver nano-particles according to any        one of the above, wherein the aliphatic hydrocarbon amine        comprises the aliphatic hydrocarbon monoamine (A) and the        aliphatic hydrocarbon monoamine (B).    -   The method for producing silver nano-particles according to any        one of the above, wherein the aliphatic hydrocarbon amine        comprises the aliphatic hydrocarbon monoamine (A) and the        aliphatic hydrocarbon diamine (C).    -   The method for producing silver nano-particles according to any        one of the above, wherein the aliphatic hydrocarbon amine        comprises the aliphatic hydrocarbon monoamine (A), the aliphatic        hydrocarbon monoamine (B), and the aliphatic hydrocarbon diamine        (C).    -   The method for producing silver nano-particles according to any        one of the above, wherein in the step of forming a complex        compound comprising the silver compound and the amine, an        aliphatic carboxylic acid is also used in addition to the        aliphatic amine.    -   The method for producing silver nano-particles according to any        one of the above, wherein in the step of forming a complex        compound comprising the silver compound and the amine, an        aliphatic carboxylic acid is not used in addition to the        aliphatic amine.

(10) Silver nano-particles produced by the method according to any oneof the above (1) to (9).

-   -   Coated silver nano-particles whose surfaces are coated with a        protective agent, wherein the coated silver nano-particles are        produced by the method according to any one of the above, and        the protective agent comprises the aliphatic amine used.    -   A silver coating composition comprising silver nano-particles        produced by the method according to any one of the above, and an        organic solvent. The silver coating composition may take any        form without any limitation. For example, a silver coating        composition in which the silver nano-particles are dispersed in        suspension state in the organic solvent (silver ink), or a        silver coating composition in which the silver nano-particles        are dispersed in kneaded state in the organic solvent (silver        paste).    -   A silver conductive material comprising:    -   a substrate, and    -   a silver conductive layer obtained by applying, onto the        substrate, a silver coating composition comprising silver        nano-particles produced by the method according to any one of        the above and an organic solvent, and calcining the silver        coating composition.

The silver conductive layer may be patterned.

The calcining is performed at a temperature of 200° C. or less, forexample, 150° C. or less, preferably 120° C. or less, for 2 hours orless, for example, 1 hour or less, preferably 30 minutes or less, morepreferably 15 minutes or less. More specifically, the calcining isperformed under conditions of about 90° C. to 120° C. and about 10minutes to 15 minutes, for example, 120° C. and 15 minutes.

-   -   A method for producing metal nano-particles comprising:    -   mixing an aliphatic hydrocarbon amine and a metal compound in        the presence of an alcohol solvent having 3 or more carbon atoms        to form a complex compound comprising the metal compound and the        amine; and    -   thermally decomposing the complex compound by heating to form        metal nano-particles.    -   The method for producing metal nano-particles, wherein the        aliphatic hydrocarbon amine comprises an aliphatic hydrocarbon        monoamine (A) comprising an aliphatic hydrocarbon group and one        amino group, said aliphatic hydrocarbon group having 6 or more        carbon atoms in total, and    -   further comprises at least one of an aliphatic hydrocarbon        monoamine (B) comprising an aliphatic hydrocarbon group and one        amino group, said aliphatic hydrocarbon group having 5 or less        carbon atoms in total; and an aliphatic hydrocarbon diamine (C)        comprising an aliphatic hydrocarbon group and two amino groups,        said aliphatic hydrocarbon group having 8 or less carbon atoms        in total.    -   Coated metal nano-particles whose surfaces are coated with a        protective agent, wherein the coated metal nano-particles are        produced by the above method, and the protective agent comprises        the aliphatic amine used.    -   A metal coating composition comprising metal nano-particles        produced by the above method, and an organic solvent. The metal        coating composition may take any form without limitation. For        example, a metal coating composition in which the metal        nano-particles are dispersed in suspension state in the organic        solvent (metal ink), or a metal coating composition in which the        metal nano-particles are dispersed in kneaded state in the        organic solvent (metal paste).

Advantageous Effects of Invention

In the present invention, when a complex compound comprising a silvercompound and an aliphatic hydrocarbon amine is formed, a silver compoundin powder form and an aliphatic hydrocarbon amine are mixed in thepresence of an alcohol solvent having 3 or more carbon atoms to formacomplex compound comprising the silver compound and the amine.Therefore, in the step of forming a complex compound, stirring can besufficiently performed and the heat of reaction generated by forming acomplex compound can be released to the outside of a reaction system.This makes it possible to provide a silver nano-particle productionmethod that is safe and simple also in terms of scaled-upindustrial-level production.

In the present invention, when an aliphatic hydrocarbon monoamine (A)having 6 or more carbon atoms in total, and at least one of an aliphatichydrocarbon monoamine (B) having 5 or less carbon atoms in total and analiphatic hydrocarbon diamine (C) having 8 or less carbon atoms in totalare used, as aliphatic amine compounds that function as acomplex-forming agent and/or a protective agent, silver nano-particleswhose surfaces are coated with these aliphatic amine compounds areformed.

The aliphatic hydrocarbon monoamine (B) and the aliphatic hydrocarbondiamine (C) each have a short carbon chain, and are therefore easilyremoved from the surfaces of the silver particles in a short time of 2hours or less, for example, 1 hour or less, preferably 30 minutes orless even by low-temperature calcining at a temperature of 200° C. orless, for example, 150° C. or less, preferably 120° C. or less. Inaddition, the presence of the monoamine (B) and/or the diamine (C)reduces the amount of the aliphatic hydrocarbon monoamine (A) adhered tothe surfaces of the silver particles. This makes it possible to easilyremove these aliphatic amine compounds from the surfaces of the silverparticles in such a short time as described above even bylow-temperature calcining at such a low temperature as described above,thereby allowing the silver particles to be sufficiently sintered.

As described above, according to the present invention, it is possibleto provide silver nano-particles that have excellent stability and candevelop excellent conductivity (low resistance value) by low-temperatureand short-time calcining even when having a silver film formed with arelatively large film thickness of, for example, 1 μm or more, and amethod for producing such silver nano-particles. In addition, accordingto the present invention, it is also possible to provide a silvercoating composition comprising the silver nano-particles in stabledispersion state in an organic solvent. Further, the present inventionis also applied to a method for producing metal nano-particlescontaining a metal other than silver, and said metal nano-particles.According to the present invention, it is possible to form a conductivefilm or a conductive line even on any plastic substrate having low heatresistance such as a PET substrate or a polypropylene substrate.

DESCRIPTION OF EMBODIMENTS

In the present invention, silver nano-particles are produced by mixingan aliphatic hydrocarbon amine and a silver compound in the presence ofan alcohol solvent having 3 or more carbon atoms to form a complexcompound comprising the silver compound and the amine, and then bythermally decomposing the complex compound by heating to form silvernano particles. Therefore, a method for producing silver nano-particlesaccording to the present invention mainly includes a complexcompound-forming step, and a thermal decomposition step of the complexcompound.

In this description, the term “nano-particles” means that primaryparticles have a size (average primary particle diameter) of less than1,000 nm. The particle size refers to the size of a particle notincluding a protective agent (a stabilizer) present on (coating) thesurface of the particle (i.e., refers to the size of silver itself). Inthe present invention, the silver nano-particles have an average primaryparticle diameter of, for example, 0.5 nm to 100 nm, preferably 0.5 nmto 50 nm, more preferably 0.5 nm to 25 nm, even more preferably 0.5 nmto 20 nm.

The silver compound used in the present invention is one that is easilydecomposed by heating to generate metallic silver. Examples of such asilver compound that can be used include: silver carboxylates such assilver formate, silver acetate, silver oxalate, silver malonate, silverbenzoate, and silver phthalate; silver halides such as silver fluoride,silver chloride, silver bromide, and silver iodide; silver sulfate,silver nitrate, silver carbonate, and the like. In terms of the factthat metallic silver is easily generated by decomposition and impuritiesother than silver are less likely to be generated, silver oxalate ispreferably used. Silver oxalate is advantageous in that silver oxalatehas a high silver content, and metallic silver is directly obtained bythermal decomposition without the need for a reducing agent, andtherefore impurities derived from a reducing agent are less likely toremain.

When metal nano-particles containing another metal other than silver areproduced, a metal compound that is easily decomposed by heating togenerate a desired metal is used instead of the silver compound. As sucha metal compound, a metal salt corresponding to the above mentionedsilver compound can be used. Examples of such a metal compound include:metal carboxylates; metal halides; and metal salt compounds such asmetal sulfates, metal nitrates, and metal carbonates. Among them, interms of the fact that a metal is easily generated by decomposition andimpurities other than a metal are less likely to be generated, metaloxalate is preferably used. Examples of another metal include Al, Au,Pt, Pd, Cu, Co, Cr, In, and Ni.

Further, in order to obtain a composite with silver, the above mentionedsilver compound and the above mentioned compound of another metal otherthan silver may be used in combination. Examples of another metalinclude Al, Au, Pt, Pd, Cu, Co, Cr, In, and Ni. The silver composite iscomposed of silver and one or more other metals, and examples thereofinclude Au—Ag, Ag—Cu, Au—Ag—Cu, Au—Ag—Pd, and the like. The amount ofsilver occupies at least 20 wt %, usually at least 50 wt %, for example,at least 80 wt % of the total amount of the metals.

In the present invention, an aliphatic hydrocarbon amine and a silvercompound are mixed in the presence of an alcohol solvent having 3 ormore carbon atoms to form a complex compound comprising the silvercompound and the amine [complex compound-forming step].

As the alcohol solvent, an alcohol having 3 to 10 carbon atoms,preferably an alcohol having 4 to 6 carbon atoms can be used. Examplesof such an alcohol include n-propanol (boiling point (bp): 97° C.),isopropanol (bp: 82° C.), n-butanol (bp: 117° C.), isobutanol (bp:107.89° C.), sec-butanol (bp: 99.5° C.), tert-butanol (bp: 82.45° C.),n-pentanol (bp: 136° C.), n-hexanol (bp: 156° C.), n-octanol (bp: 194°C.), 2-octanol (bp: 174° C.), and the like. Among them, butanolsselected from n-butanol, isobutanol, sec-butanol and tert-butanol, andhexanols are preferred in consideration of the fact that the temperatureof the thermal decomposition step of the complex compound subsequentlyperformed can be increased, and post-treatment after the formation ofsilver nano-particles is easy. Particularly, n-butanol and n-hexanol arepreferred.

In order to sufficiently stir a silver compound-alcohol slurry, thealcohol solvent is used in an amount of, for example, 120 parts byweight or more, preferably 130 parts by weight or more, more preferably150 parts by weight or more with respect to 100 parts by weight of thesilver compound. The upper limit of the amount of the alcohol-basedsolvent is not particularly limited, and is, for example, 1,000 parts byweight or less, preferably 800 parts by weight or less, more preferably500 parts by weight or less with respect to 100 parts by weight of thesilver compound.

In the present invention, the mixing of an aliphatic hydrocarbon amineand a silver compound in the presence of an alcohol solvent having 3 ormore carbon atoms can be performed in several ways.

For example, the mixing may be performed by first mixing a solid silvercompound and an alcohol solvent to obtain a silver compound-alcoholslurry [slurry-forming step], and then by adding an aliphatichydrocarbon amine to the obtained silver compound-alcohol slurry. Theslurry represents a mixture in which the solid silver compound isdispersed in the alcohol solvent. The slurry may be obtained by addingthe alcohol solvent to the solid silver compound contained in a reactioncontainer.

Alternatively, the silver compound-alcohol slurry may be added to thealiphatic hydrocarbon amine and the alcohol solvent contained in areaction container.

In the present invention, as an aliphatic hydrocarbon amine thatfunctions as a complex-forming agent and/or a protective agent, forexample, one is used, which contains an aliphatic hydrocarbon monoamine(A) having a hydrocarbon group having 6 or more carbon atoms in total,and further contains at least one of an aliphatic hydrocarbon monoamine(B) comprising an aliphatic hydrocarbon group and one amino group, saidaliphatic hydrocarbon group having 5 or less carbon atoms in total; andan aliphatic hydrocarbon diamine (C) comprising an aliphatic hydrocarbongroup and two amino groups, said aliphatic hydrocarbon group having 8 orless carbon atoms in total. These respective components are usually usedin the form of a liquid amine mixture, but mixing of the amines with thesilver compound (or alcohol slurry thereof) does not always need to beperformed using a mixture of the amines. These amines may be added oneby one to the silver compound (or its alcohol slurry thereof).

Although established, the “aliphatic hydrocarbon monoamine” in thisdescription refers to a compound composed of one to three monovalentaliphatic hydrocarbon groups and one amino group. The “hydrocarbongroup” refers to a group only composed of carbon and hydrogen. However,if necessary, each of the aliphatic hydrocarbon monoamine (A) and thealiphatic hydrocarbon monoamine (B) may have, on its hydrocarbon group,a substituent group containing a hetero atom (atom other than carbon andhydrogen) such as an oxygen atom or a nitrogen atom. This nitrogen atomdoes not constitute an amino group.

Further, the “aliphatic hydrocarbon diamine” refers to a compoundcomposed of a bivalent aliphatic hydrocarbon group (alkylene group), twoamino groups between which said aliphatic hydrocarbon group isinterposed, and, if necessary, aliphatic hydrocarbon group(s) (alkylgroup(s)) substituted for hydrogen atom(s) on the amino group(s).However, if necessary, the aliphatic hydrocarbon diamine (C) may have,on its hydrocarbon′ group, a substituent group containing a hetero atom(atom other than carbon and hydrogen) such as an oxygen atom or anitrogen atom. This nitrogen atom does not constitute an amino group.

The aliphatic hydrocarbon monoamine (A) having 6 or more carbon atoms intotal has, due to its hydrocarbon chain, high performance as aprotective agent (a stabilizer) onto the surfaces of resulting silverparticles.

The aliphatic hydrocarbon monoamine (A) includes a primary amine, asecondary amine, and a tertiary amine. Examples of the primary amineinclude saturated aliphatic hydrocarbon monoamines (i.e.,alkylmonoamines) such as hexylamine, heptylamine, octylamine,nonylamine, decylamine, undecylamine, dodecylamine, tridecylamine,tetradecylamine, pentadecylamine, hexadecylamine, heptadecylamine, andoctadecylamine. Examples of the saturated aliphatic hydrocarbonmonoamine other than the above-mentioned linear aliphatic monoaminesinclude branched aliphatic hydrocarbon amines such as isohexylamine,2-ethylhexylamine, and tert-octylamine. Another example of the saturatedaliphatic hydrocarbon monoamine includes cyclohexylamine. Other examplesof the primary amine include unsaturated aliphatic hydrocarbonmonoamines (i.e., alkenylmonoamines) such as oleylamine.

Examples of the secondary amine include dialkylmonoamines such asN,N-dipropylamine, N,N-dibutylamine, N,N-dipentylamine,N,N-dihexylamine, N,N-dipeptylamine, N,N-dioctylamine, N,N-dinonylamine,N,N-didecylamine, N,N-diundecylamine, N,N-didodecylamine,N-methyl-N-propylamine, N-ethyl-N-propylamine, andN-propyl-N-butylamine. Examples of the tertiary amine includetributylamine, trihexylamine, and the like.

Among them, saturated aliphatic hydrocarbon monoamines having 6 or morecarbon atoms are preferred. When the number of carbon atoms is 6 ormore, space can be secured between silver particles by adsorption ofamino groups to the surfaces of the silver particles, thereby improvingthe effect of preventing agglomeration of the silver particles. Theupper limit of the number of carbon atoms is not particularly limited,but saturated aliphatic monoamines having up to 18 carbon atoms areusually preferred in consideration of ease of availability, ease ofremoval during calcining, etc. Particularly, alkylmonoamines having 6 to12 carbon atoms such as hexylamine, heptylamine, octylamine, nonylamine,decylamine, undecylamine, and dodecylamine are preferably used. Theabove-mentioned aliphatic hydrocarbon monoamines (A) may be used singlyor in combination of two or more of them.

The aliphatic hydrocarbon monoamine (B) having 5 or less carbon atoms intotal has a shorter carbon chain than the aliphatic monoamine (A) having6 or more carbon atoms in total, and therefore the function of thealiphatic hydrocarbon monoamine (B) itself as a protective agent (astabilizer) is considered to be low. However, the aliphatic hydrocarbonmonoamine (B) has a high ability to coordinate to silver in the silvercompound due to its higher polarity than the aliphatic monoamine (A),and is therefore considered to have the effect of promoting complexformation. In addition, the aliphatic hydrocarbon monoamine (B) has ashort carbon chain, and therefore can be removed from the surfaces ofsilver particles in a short time of 30 minutes or less, or 20 minutes orless, even by low-temperature calcining at a temperature of, forexample, 120° C. or less, or about 100° C. or less, which is effectivefor low-temperature calcining of resulting silver nano-particles.

Examples of the aliphatic hydrocarbon monoamine (B) include saturatedaliphatic hydrocarbon monoamines (i.e., alkylmonoamines) having 2 to 5carbon atoms such as ethylamine, n-propylamine, isopropylamine,n-butylamine, isobutylamine, sec-butylamine, tert-butylamine,pentylamine, isopentylamine, and tert-pentylamine. Other examples of thealiphatic hydrocarbon monoamine (B) include dialkylmonoamines such asN,N-dimethylamine and N,N-diethylamine.

Among them, n-butylamine, isobutylamine, sec-butylamine,tert-butylamine, pentylamine, isopentylamine, tert-pentylamine, and thelike are preferred, and the above-mentioned butylamines are particularlypreferred. The above-mentioned aliphatic hydrocarbon monoamines (B) maybe used singly or in combination of two or more of them.

The aliphatic hydrocarbon diamine (C) having 8 or less carbon atoms intotal has a high ability to coordinate to silver in the silver compound,and therefore has the effect of promoting complex formation. Generally,aliphatic hydrocarbon diamines have higher polarity than aliphatichydrocarbon monoamines, and therefore have a high ability to coordinateto silver in a silver compound. Further, the aliphatic hydrocarbondiamine (C) has the effect of promoting lower-temperature andshorter-time thermal decomposition in the thermal-decomposition step ofthe complex compound, and therefore production of silver nano-particlescan be more efficiently conducted. Further, a protective film containingthe aliphatic diamine (C) on silver particles has high polarity, whichimproves the dispersion stability, of the silver particles in adispersion medium comprising a highly-polar solvent. Furthermore, thealiphatic diamine (C) has a short carbon chain, and therefore can beremoved from the surfaces of silver particles in a short time of 30minutes or less, or 20 minutes or less, even by low-temperaturecalcining at a temperature of, for example, 120° C. or less, or about100° C. or less, which is effective for low-temperature and short-timecalcining of resulting silver nano-particles.

The aliphatic hydrocarbon diamine (C) is not particularly limited, andexamples thereof include ethylenediamine, N,N-dimethylethylenediamine,N,N′-dimethylethylenediamine, N,N-diethylethylenediamine,N,N′-diethylethylenediamine, 1,3-propanediamine,2,2-dimethyl-1,3-propanediamine, N,N-dimethyl-1,3-propanediamine,N,N′-dimethyl-1,3-propanediamine, N,N-diethyl-1,3-propanediamine,N,N′-diethyl-1,3-propanediamine, 1,4-butanediamine,N,N-dimethyl-1,4-butanediamine, N,N′-dimethyl-1,4-butanediamine,N,N-diethyl-1,4-butanediamine, N,N′-diethyl-1,4-butanediamine,1,5-pentanediamine, 1,5-diamino-2-methylpentane, 1,6-hexanediamine,N,N-dimethyl-1,6-hexanediamine, N,N′-dimethyl-1,6-hexanediamine,1,7-heptanediamine, 1,8-octanediamine, and the like. They are allalkylenediamines having 8 or less carbon atoms in total in which atleast one of the two amino groups is a primary amino group or asecondary amino group, and have a high ability to coordinate to silverin the silver compound, and therefore have the effect of promotingcomplex formation.

Among them, N,N-dimethylethylenediamine, N,N-diethylethylenediamine,N,N-dimethyl-1,3-propanediamine, N,N-diethyl-1,3-propanediamine,N,N-dimethyl-1,4-butanediamine, N,N-diethyl-1,4-butanediamine,N,N-dimethyl-1,6-hexanediamine, and the like are preferred, which arealkylenediamines having 8 or less carbon atoms in total in which one ofthe two amino groups is a primary amino group (—NH₂) and the other is atertiary amino group (—NR¹R²). Such preferred alkylenediamines arerepresented by the following structural formula:R¹R²N—R—NH₂

wherein R represents a bivalent alkylene group, R¹ and R² may be thesame or different from each other and each represent an alkyl group, andthe total number of carbon atoms of R, R¹, and R² is 8 or less. Thealkylene group does not usually contain a hetero atom (atom other thancarbon and hydrogen) such as an oxygen atom or a nitrogen atom, but ifnecessary, may have a substituent group containing such a hetero atom.Further, the alkyl group does not usually contain a hetero atom such asan oxygen atom or a nitrogen atom, but if necessary, may have asubstituent group containing such a hetero atom.

When one of the two amino groups is a primary amino group, the abilityto coordinate to silver in the silver compound is high, which isadvantageous for complex formation, and when the other is a tertiaryamino group, a resulting complex is prevented from having a complicatednetwork structure because a tertiary amino group has a poor ability tocoordinate to a silver atom. If a complex has a complicated networkstructure, there is a case where the thermal-decomposition step of thecomplex requires a high temperature. Among these diamines, those having6 or less carbon atoms in total are preferred, and those having 5 orless carbon atoms in total are more preferred in terms of the fact thatthey can be removed from the surfaces of silver particles in a shorttime even by low-temperature calcining. The above-mentioned aliphatichydrocarbon diamines (C) may be used singly or in combination of two ormore of them.

The ratio between the aliphatic hydrocarbon monoamine (A) having 6 ormore carbon atoms in total, and one or both of the aliphatic hydrocarbonmonoamine (B) having 5 or less carbon atoms in total and the aliphatichydrocarbon diamine (C) having 8 or less carbon atoms in total used inthe present invention is not particularly limited. For example,

the amount of the aliphatic monoamine (A) may be 5 mol % to 65 mol %;and

the total amount of the aliphatic monoamine (B) and the aliphaticdiamine (C) may be 35 mol % to 95 mol %,

on the basis of the total amount of the amines [(A)+(B)+(C)]. By settingthe content of the aliphatic monoamine (A) to 5 mol % to 65 mol %, thecarbon chain of the component (A), can easily fulfill its function ofprotecting and stabilizing the surfaces of resulting silver particles.If the content of the component (A) is less than 5 mol %, there is acase where the protective and stabilization function is poorlydeveloped. On the other hand, if the content of the component (A)exceeds 65 mol %, the protective and stabilization function issufficient, but the component (A) is poorly removed by low-temperaturecalcining.

When the aliphatic monoamine (A), and further both the aliphaticmonoamine (B) and the aliphatic diamine (C) are used, the ratio amongthem used is not particularly limited. For example,

the amount of the aliphatic monoamine (A) may be 5 mol % to 65 mol %;

the amount of the aliphatic monoamine (B) may be 5 mol % to 70 mol %;and

the amount of the aliphatic diamine (C) may be 5 mol % to 50 mol %, onthe basis of the total amount of the amines [(A)+(B)+(C)].

In this case, the lower limit of the content of the component (A) ispreferably 10 mol % or more, more preferably 20 mol % or more. The upperlimit of the content of the component (A) is preferably 65 mol % orless, more preferably 60 mol % or less.

By setting the content of the aliphatic monoamine (B) to 5 mol % to 70mol %, the effect of promoting complex formation is easily obtained, thealiphatic monoamine (B) itself can contribute to low-temperature andshort-time calcining, and the effect of facilitating the removal of thealiphatic diamine (C) from the surfaces of silver particles duringcalcining is easily obtained. If the content of the component (B) isless than 5 mol %, there is a case where the effect of promoting complexformation is poor, or the component (C) is poorly removed from thesurfaces of silver particles during calcining. On the other hand, if thecontent of the component (B) exceeds 70 mol %, the effect of promotingcomplex formation is obtained, but the content of the aliphaticmonoamine (A) is relatively reduced so that the surfaces of resultingsilver particles are poorly protected and stabilized. The lower limit ofthe content of the component (B) is preferably 10 mol % or more, morepreferably 15 mol % or more. The upper limit of the content of thecomponent (B) is preferably 65 mol % or less, more preferably 60 mol %or less.

By setting the content of the aliphatic diamine (C) to 5 mol % to 50 mol%, the effect of promoting complex formation and the effect of promotingthe thermal-decomposition of the complex are easily obtained, andfurther, the dispersion stability of silver particles in a dispersionmedium containing a highly-polar solvent is improved because aprotective film containing the aliphatic diamine (C) on silver particleshas high polarity. If the content of the component (C) is less than 5mol %, there is a case where the effect of promoting complex formationand the effect of promoting the thermal-decomposition of the complex arepoor. On the other hand, if the content of the component (C) exceeds 50mol %, the effect of promoting complex formation and the effect ofpromoting the thermal-decomposition of the complex are obtained, but thecontent of the aliphatic monoamine (A) is relatively reduced so that thesurfaces of resulting silver particles are poorly protected andstabilized. The lower limit of the content of the component (C) ispreferably 5 mol % or more, more preferably 10 mol % or more. The upperlimit of the content of the component (C) is preferably 45 mol % orless, more preferably 40 mol % or less.

When the aliphatic monoamine (A) and the aliphatic monoamine (B) areused (without using the aliphatic diamine (C)), the ratio between themused is not particularly limited. For example, in consideration of theabove-described functions of these components,

the amount of the aliphatic monoamine (A) may be 5 mol % to 65 mol %;and

the amount of the aliphatic monoamine (B) may be 35 mol % to 95 mol %,

on the basis of the total amount of the amines. [(A)+(B)].

When the aliphatic monoamine (A) and the aliphatic diamine (C) are used(without using the aliphatic monoamine (B)), the ratio between them usedis not particularly limited. For example, in consideration of theabove-described functions of these components,

the amount of the aliphatic monoamine (A) may be 5 mol % to 65 mol %;and

the amount of the aliphatic diamine (C) may be 35 mol % to 95 mol %,

on the basis of the total amount of the amines [(A)+(C)].

The above ratios among/between the aliphatic monoamine (A) and thealiphatic monoamine (B) and/or the aliphatic diamine (C) used areexamples and may be changed in various manners.

In the present invention, the use of the aliphatic monoamine (B) and/orthe aliphatic diamine (C) each having a high ability to coordinate tosilver in the silver compound makes it possible, depending on theircontents, to reduce the amount of the aliphatic monoamine (A) having 6or more carbon atoms in total adhered to the surfaces of silverparticles. Therefore, these aliphatic amine compounds are easily removedfrom the surfaces of silver particles even by the above-describedlow-temperature and short-time calcining so that the silver particlesare sufficiently sintered.

In the present invention, the total amount of the aliphatic hydrocarbonamine [e.g., (A) and (B) and/or (C)] is not particularly limited, butmay be about 1 to 50 moles per 1 mole of silver atoms in the silvercompound as a starting material. If the total amount of the aminecomponents [(A) and (B) and/or (C)] is less than 1 mole per 1 mole ofthe silver atoms, there is a possibility that part of the silvercompound remains without being converted to a complex compound in thecomplex compound-forming step so that, in the subsequent thermaldecomposition step, silver particles have poor uniformity and becomeenlarged or the silver compound remains without being thermallydecomposed. On the other hand, it is considered that even when the totalamount of the amine components [(A) and (B) and/or (C)] exceeds about 50moles per 1 mole of the silver atoms, there are few advantages. Bysetting the total amount of the amine components to about 1 to 50 moles,the complex compound-forming step and the thermal-decomposition step ofthe complex compound can be successfully performed. The lower limit ofthe total amount of the amine components is preferably 2 mol or more,more preferably 6 mol or more per 1 mole of silver atoms in the silvercompound. It is to be noted that the molecule of silver oxalate containstwo silver atoms.

In the present invention, an aliphatic carboxylic acid (D) may furtherbe used as a stabilizer to further improve the dispersibility of silvernano-particles in a dispersion medium. The aliphatic carboxylic acid (D)may be used together with the above-described amines, and may be used byadding to the liquid amine mixture. The use of the aliphatic carboxylicacid (D) may improve the stability of silver nano-particles, especiallythe stability of silver nano-particles in a coating material state wherethe silver nano-particles are dispersed in an organic solvent.

As the aliphatic carboxylic acid (D), a saturated or unsaturatedaliphatic carboxylic acid is used. Examples of the aliphatic carboxylicacid include saturated aliphatic monocarboxylic acids having 4 or morecarbon atoms such as butanoic acid, pentanoic acid, hexanoic acid,heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoicacid, dodecanoic acid, tridecanoic acid, tetradecanoic acid,pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoicacid, nonadecanoic acid, icosanoic acid, and eicosenoic acid; andunsaturated aliphatic monocarboxylic acids having 8 or more carbon atomssuch as oleic acid, elaidic acid, linoleic acid, and palmitoleic acid.

Among them, saturated or unsaturated aliphatic monocarboxylic acidshaving 8 to 18 carbon atoms are preferred. When the number of carbonatoms is 8 or more, space can be secured between silver particles byadsorption of carboxylic groups to the surfaces of the silver particles,thereby improving the effect of preventing agglomeration of the silverparticles. In consideration of ease of availability, ease of removalduring calcining, etc., saturated or unsaturated aliphaticmonocarboxylic compounds having up to 18 carbon atoms are usuallypreferred. Particularly, octanoic acid, oleic acid, and the like arepreferably used. The above-mentioned aliphatic carboxylic acids (D) maybe used singly or in combination of two or more of them.

When the aliphatic carboxylic acid (D) is used, the amount of thealiphatic carboxylic acid (D) used may be, for example, about 0.05 to 10moles, preferably 0.1 to 5 moles, more preferably 0.5 to 2 moles per 1mole of silver atoms in the silver compound as a starting material. Ifthe amount of the component (D) is less than 0.05 moles per 1 mole ofthe silver atoms, the effect of improving dispersion stability obtainedby adding the component (D) is poor. On the other hand, if the amount ofthe component (D) reaches 10 moles, the effect of improving dispersionstability is saturated and the component (D) is poorly removed bylow-temperature calcining. It is to be noted that the aliphaticcarboxylic, acid (D) does not necessarily need to be used.

In the present invention, an amine mixture liquid containing thealiphatic monoamine (A) and further one or both of the aliphaticmonoamine (B) and the aliphatic diamine (C) is usually prepared[preparation step for amine mixture liquid].

The amine mixture liquid can be prepared by stirring the amine component(A), the amine component (B) and/or the amine component (C), and ifused, the carboxylic acid component (D) in a given ratio at a roomtemperature.

The aliphatic hydrocarbon amine mixture liquid containing the respectiveamine components is added to the silver compound (or alcohol slurrythereof) to form a complex compound containing the silver compound andthe amine [complex compound-forming step]. The amine components may beadded to the silver compound (or alcohol slurry thereof) one by onewithout using a mixture liquid thereof.

When metal nano-particles containing another metal other than silver areproduced, a metal compound containing a desired metal (or alcohol slurrythereof) is used instead of the silver compound (or alcohol slurrythereof).

The silver compound (or alcohol slurry thereof) or the metal compound(or alcohol slurry thereof), and a given amount of the amine mixtureliquid are mixed. At this time, the mixing may be performed by stirringthem at a room temperature, or may be performed by stirring them while amixture of them is appropriately cooled to a room temperature or lessbecause the coordination reaction of the amines to the silver compound(or the metal compound) is accompanied by heat generation. The mixing ofthe silver compound and the amine mixture liquid is performed in thepresence of an alcohol, and therefore stirring and cooling can besuccessfully performed. The alcohol and excess amines function as areaction medium.

In a conventional method for thermally decomposing a silver-aminecomplex, a liquid aliphatic amine component is first placed in areaction container, and then a powder silver compound (silver oxalate)is added thereto. The liquid aliphatic amine component is flammable, andtherefore addition of the powder silver compound to the liquid aliphaticamine compound is dangerous. That is, there is a risk of ignition due tostatic electricity generated by addition of the powder silver compound.Further, there is also a risk of a runaway exothermic reaction due to acomplex-forming reaction locally caused by addition of the powder silvercompound. According to the present invention, such risks can be avoided.

When a complex compound is formed, the formed complex compound generallyexhibits a color corresponding to its components, and therefore theendpoint of a complex compound-forming reaction can be determined bydetecting the end of a change in the color of a reaction mixture by anappropriate spectroscopic method or the like. A complex compound formedfrom silver oxalate is generally colorless (appears white to our eyes),but even in such a case, it is possible to determine the state offormation of a complex compound based on a change in the form of areaction mixture such as a change in viscosity. In this way, asilver-amine complex (or a metal-amine complex) is obtained in a mediummainly containing the alcohol and the amines.

Then, the obtained complex compound is thermally decomposed by heatingto form silver nano-particles [thermal-decomposition step of complexcompound]. When a metal compound containing another metal other thansilver is used, desired metal nano-particles are formed. The silvernano-particles (metal nano-particles) are formed without using areducing agent. However, if necessary, an appropriate reducing agent maybe used without impairing the effects of the present invention.

In such a metal-amine complex decomposition method, the amines generallyplay a role in controlling the mode of formation of microparticles byagglomeration of an atomic metal generated by decomposition of the metalcompound, and in forming film on the surfaces of the formed metalmicroparticles to prevent reagglomeration of the microparticles. Thatis, it is considered that when the complex compound of the metalcompound and the amine is heated, the metal compound is thermallydecomposed to generate an atomic metal while the coordination bond ofthe amine to a metallic atom is maintained, and then the metallic atomscoordinated with the amine are agglomerated to form metal nano-particlescoated with an amine protective film.

At this time, the thermal decomposition is preferably performed bystirring the complex compound in a reaction medium mainly containing thealcohol and the amines. The thermal decomposition may be performed in atemperature range in which coated silver nano-particles (or coated metalnano-particles) are formed, but from the viewpoint of preventing theelimination of the amine from the surfaces of silver particles (or fromthe surfaces of metal particles), the thermal decomposition ispreferably performed at a temperature as low as possible within such atemperature range. In case of the complex compound from silver oxalate,the thermal decomposition temperature may be, for example, about 80° C.to 120° C., preferably about 95° C. to 115° C., more specifically about100° C. to 110° C. In case of the complex compound from silver oxalate,heating at about 100° C. allows decomposition and reduction of silverions to occur so that coated silver nano-particles can be obtained.Further, the thermal decomposition of silver oxalate itself generallyoccurs at about 200° C. The reason why the thermal decompositiontemperature of a silver oxalate-amine complex compound is about 100° C.lower than that of silver oxalate itself is not clear, but it isestimated that a coordination polymer structure formed by pure silveroxalate is broken by forming a complex compound of silver oxalate withthe amine.

Further, the thermal decomposition of the complex compound is preferablyperformed in an inert gas atmosphere such as argon, but may be performedin the atmosphere.

When the complex compound is thermally decomposed, a suspensionexhibiting a glossy blue color is obtained. Then, the alcohol solvent,the excess amines, etc. are removed from the suspension by, for example,sedimentation of silver nano-particles (or metal nano-particles) anddecantation and washing with an appropriate solvent (water or an organicsolvent) to obtain desired stable coated silver nano-particles (orcoated metal nano-particles) [silver nano-particle post-treatment step].After the washing, the coated silver nano-particles are dried to obtaina powder of the desired stable coated silver nano-particles (or coatedmetal nano-particles).

The decantation and washing are performed using water or an organicsolvent. Examples of the organic solvent that may be used includealiphatic hydrocarbon solvents such as pentane, hexane, heptane, octane,nonane, decane, undecane, dodecane, tridecane, and tetradecane;alicyclic hydrocarbon solvents such as methylcyclohexane andcyclohexane; aromatic hydrocarbon solvents such as toluene, xylene, andmesitylene; alcohol solvents such as methanol, ethanol, propanol, andbutanol; acetonitrile; and mixed solvents of them.

The step of forming the silver nano-particles according to the presentinvention does not require the use of a reducing agent. Therefore, aby-product derived from a reducing agent is not formed, coated silvernano-particles are easily separated from a reaction system, andhigh-purity coated silver nano-particles are obtained. However, ifnecessary, an appropriate reducing agent may be used without impairingthe effects of the present invention.

In this way, silver nano-particles whose surfaces are coated with aprotective agent used are formed. The protective agent comprises, forexample, the aliphatic monoamine (A), and further one or both of thealiphatic monoamine (B) and the aliphatic diamine (C), and further ifused, the carboxylic acid (D). The ratio among/between them contained inthe protective agent is the same as the ratio among/between them used inthe amine mixture liquid. The same goes for the metal nano-particles.

A silver coating composition can be prepared using the obtained silvernano-particles. The silver coating composition can take any form withoutany limitation. For example, a silver coating composition called “silverink” can be prepared by dispersing the silver nano-particles insuspension state in an appropriate organic solvent (dispersion medium).Alternatively, a silver coating composition called “silver paste” can beprepared by dispersing the silver nano-particles in kneaded state in anorganic solvent. Examples of the organic solvent used to obtain a silvercoating composition include: aliphatic hydrocarbon solvents such aspentane, hexane, heptane, octane, nonane, decane, undecane, dodecane,tridecane, and tetradecane; alicyclic hydrocarbon solvents such ascyclohexane and methylcyclohexane; aromatic hydrocarbon solvents such astoluene, xylene, and mesitylene; and alcohol solvents such as methanol,ethanol, propanol, n-butanol, n-pentanol, n-hexanol, n-heptanol,n-octanol, n-nonanol, and n-decanol. Examples of the organic solventused to obtain a silver paste as a silver coating composition includeterpene-based solvents such as terpineol and dihydroterpineol. The kindand amount of organic solvent used may be appropriately determineddepending on a desired concentration or viscosity of the silver coatingcomposition (silver ink, silver paste). The same goes for the metalnano-particles.

The silver nano-particle powder and the silver coating compositionobtained in the present invention have excellent stability. For example,the silver nano-particle powder is stable during storage at a roomtemperature for 1 month or more. The silver coating composition isstable at a silver concentration of, for example, 50 wt % at a roomtemperature for 1 month or more without the occurrence of agglomerationand fusion.

The prepared silver coating composition is applied onto a substrate andis then calcined.

The application can be performed by a known method such as spin coating,inkjet printing, screen printing, dispenser printing, relief printing(flexography), dye sublimation printing, offset printing, laser printerprinting (toner printing), intaglio printing (gravure printing), contactprinting, or microcontact printing. By using such a printing technique,a patterned silver coating composition layer is obtained, and apatterned silver conductive layer is obtained by calcining.

The calcining can be performed at 200° C. or less, for example, a roomtemperature (25° C.) or more and 150° C. or less, preferably a roomtemperature (25° C.) or more and 120° C. or less. However, in order tocomplete the sintering of silver by short-time calcining, the calciningmay be performed at a temperature of 60° C. or more and 200° C. or less,for example, 80° C. or more and 150° C. or less, preferably 90° C. ormore and 120° C. or less. The time of calcining may be appropriatelydetermined in consideration of the amount of a silver ink applied, thecalcining temperature, etc., and may be, for example, several hours(e.g., 3 hours, or 2 hours) or less, preferably 1 hour or less, morepreferably 30 minutes or less, even more preferably 10 minutes to 20minutes, more specifically 10 minutes to 15 minutes.

The silver nano-particles have such a constitution as described above,and are therefore sufficiently sintered even by such low-temperature andshort-time calcining. As a result, excellent conductivity (lowresistance value) is developed. A silver conductive layer having a lowresistance value (e.g., 15 μΩcm or less, in the range of 7 to 15 μΩcm)is formed. The resistance value of bulk silver is 1.6 μΩcm.

Since the calcining can be performed at a low temperature, not only aglass substrate or a heat-resistant plastic substrate such as apolyimide-based film but also a general-purpose plastic substrate havinglow heat resistance, such as a polyester-based film, e.g., apolyethylene terephthalate (PET) film and a polyethylene naphthalate(PEN) film, or a polyolefin-based film, e.g., polypropylene film, can besuitably used as a substrate. Further, short-time calcining reduces theload on such a general-purpose plastic substrate having low heatresistance, and improves production efficiency.

The silver conductive material according to the present invention can beapplied to electromagnetic wave control materials, circuit boards,antennas, radiator plates, liquid crystal displays, organic EL displays,field emission displays (FEDs), IC cards, IC tags, solar cells, LEDdevices, organic transistors, condensers (capacitors), electronic paper,flexible batteries, flexible sensors, membrane switches, touch panels,EMI shields, and the like.

The thickness of the silver conductive layer may be appropriatelydetermined depending on the intended use. Particularly, the use of thesilver nano-particles according to the present invention makes itpossible, even when a silver conductive layer having a relatively largefilm thickness is formed, for the silver conductive layer to have highconductivity. The thickness of the silver conductive layer may beselected from the range of, for example, 5 nm to 10 μm, preferably 100nm to 5 μm, more preferably 300 nm to 2 μm.

The present invention has been described above with reference mainly tosilver nano-particles, but is applied also to a method for producingmetal nano-particles containing a metal other than silver and said metalnano-particles.

EXAMPLES

Hereinbelow, the present invention will be described more specificallywith reference to examples, but is not limited to these examples.

[Specific Resistance Value of Calcined Silver Film]

The specific resistance value of an obtained calcined silver film wasmeasured by a four-terminal method (Loresta GP MCP-T610). The measuringlimit of this device is 10⁷ Ωcm.

Reagents used in Examples and Comparative Example are as follows:

n-Butylamine (MW: 73.14): reagent manufactured by Tokyo ChemicalIndustry Co., Ltd.;

n-Hexylamine (MW: 101.19): reagent manufactured by Tokyo ChemicalIndustry Co., Ltd.;

n-Octylamine (MW: 129.25): reagent manufactured by Tokyo ChemicalIndustry Co., Ltd.;

Dodecylamine (MW: 185.35): reagent manufactured by Wako Pure ChemicalIndustries, Ltd.;

N,N-Dimethyl-1,3-propanediamine (MW: 102.18): reagent manufactured byTokyo Chemical Industry Co., Ltd.;

Methanol: special grade reagent manufactured by Wako Pure ChemicalIndustries, Ltd.;

1-Butanol: special grade reagent manufactured by Wako Pure ChemicalIndustries, Ltd.;

tert-Butanol: reagent manufactured by Wako Pure Chemical Industries,Ltd.;

1-Hexanol: reagent manufactured by Wako Pure Chemical Industries, Ltd.;

Decalin: reagent manufactured by Tokyo Chemical Industry Co., Ltd.;

Decane: reagent manufactured by Wako Pure Chemical Industries, Ltd.; and

Silver oxalate (MW: 303.78): synthesized from silver nitrate(manufactured by Wako Pure Chemical Industries, Ltd.) and oxalic aciddihydrate (manufactured by Wako Pure Chemical Industries, Ltd.).

Example 1

(Preparation of Silver Nano-Particles)

In a 100-mL flask, 3.0 g (9.9 mmol) of silver oxalate was placed, andthen 4.5 g of 1-butanol was added and the resulting mixture was stirredat a room temperature to prepare a slurry of silver oxalate in1-butanol.

A amine mixture liquid of 8.67 g (118.5 mmol) of n-butylamine, 6.00 g(59.3 mmol) of n-hexylamine, 5.74 g (44.4 mmol) of n-octylamine, 2.75 g(14.8 mmol) of dodecylamine, and 6.05 g (59.3 mmol) ofN,N-dimethyl-1,3-propanediamine was dropped into the slurry of silveroxalate in 1-butanol at 30° C. After the completion of the dropping, theresulting mixture was stirred at 30° C. for 2 hours to proceed acomplex-forming reaction of a silver oxalate with amine, thereby forminga white substance (silver oxalate-amine complex).

After the silver oxalate-amine complex was formed, the reaction mixturewas heated with stirring and, as a result, was brought into a refluxstate at 100° C., and the temperature of the reaction mixture did notrise any further. The silver oxalate-amine complex was thermallydecomposed at this reflux temperature of 100° C. to obtain a suspensionin which deep blue silver nano-particles were suspended in the aminemixture liquid.

In both the complex-forming reaction step and the thermal decompositionstep, stirring could be very successfully performed.

Then, the obtained suspension was cooled and 30 g of methanol was addedthereto with stirring, and then the silver nano-particles were spun downby centrifugation to remove a supernatant. Then, 9 g of methanol wasagain added to the silver nano-particles with stirring, and then thesilver nano-particles were spun down by centrifugation to remove asupernatant. In this way, wet silver nano-particles were obtained.

(Preparation and Calcining of Nano-Silver Coating Material)

Then, a mixed solvent of 1-hexanol/decalin (weight ratio=1/1) was addedto the wet silver nano-particles with stirring so that a silverconcentration was 40 wt % to prepare a silver nano-particle dispersionliquid. This silver nano-particle dispersion liquid was applied onto analkali-free glass plate by spin coating to forma coating film whose filmthickness after calcining was about 0.5 μm to 1.0 μm.

After being formed, the coating film was immediately calcined in a fandrying oven in a condition at 120° C. for 15 minutes to form a calcinedsilver film having a thickness of about 0.8 μm. The specific resistancevalue of the obtained calcined silver film was measured by afour-terminal method and found to be 7.0 μΩcm.

(Regarding Silver Oxalate-Amine Complex)

The white substance obtained in the process of preparing silvernano-particles was analyzed by a DSC (differential scanningcalorimeter), and as a result, its average exothermic onset temperatureby thermal decomposition was 102.5° C. On the other hand, silver oxalateas a starting material was also analyzed by a DSC similarly, and as aresult, its average exothermic onset temperature by thermaldecomposition was 218° C. That is, the white substance obtained in theprocess of preparing silver nano-particles had a lower thermaldecomposition temperature than silver oxalate as a starting material.The results indicate that the white substance obtained in the process ofpreparing silver nano-particles was a material obtained by bondingbetween silver oxalate and the alkylamine, and the white substance wasestimated to be a silver oxalate-amine complex in which the amino groupof the alkylamine was coordinated to a silver atom in silver oxalate.

The DSC analysis was performed under the following conditions:

Device: DSC 6220-ASD2 (manufactured by SII Nanotechnology Inc.);

Sample container: 15 μL gold-plated sealed cell (manufactured by SIINanotechnology Inc.);

Temperature rise rate: 10° C./min (room temperature to 600° C.);Atmosphere gas inside the cell: air filled at atmospheric pressure; and

Atmosphere gas outside the cell: nitrogen stream (50 mL/min).

In addition, the IR spectrum of the white substance obtained in theprocess of preparing silver nano-particles was measured, and as aresult, absorption derived from the alkyl group of the alkylamine wasobserved (at about 2,900 cm⁻¹ and 1,000 cm⁻¹). The result also indicatesthat the viscous white substance obtained in the process of preparingsilver nano-particles was a material obtained by bonding between silveroxalate and the alkylamine, and the white substance was estimated to bea silver oxalate-amine complex in which an amino group was coordinatedto a silver atom in silver oxalate.

Example 2

In preparation of silver nano-particles, a slurry of silver oxalate in1-hexanol was prepared in the same manner as in Example 1 except that4.5 g of 1-hexanol was used instead of 1-butanol for 3.0 g (9.9 mmol) ofsilver oxalate. Then, in the same manner as in Example 1, the aminemixture liquid was dropped into the obtained slurry of silver oxalate in1-hexanol to form a white substance (silver oxalate-amine complex).

After the silver oxalate-amine complex was formed, the reaction mixturewas heated with stirring and, as a result, was brought into a refluxstate at 102° C., and the temperature of the reaction mixture did notrise any further. The silver oxalate-amine complex was thermallydecomposed at this reflux temperature of 102° C. to obtain a suspensionin which deep blue silver nano-particles were suspended in the aminemixture liquid. In both the complex-forming reaction step and thethermal decomposition step, stirring could be very successfullyperformed.

Then, in the same manner as in Example 1, a silver nano-particledispersion liquid was prepared using the obtained suspension, and acalcined silver film having a thickness of about 0.6 μm was formed. Thespecific resistance value of the obtained calcined silver film wasmeasured by a four-terminal method and found to be 15.4 μΩcm.

Example 3

In preparation of silver nano-particles, a slurry of silver oxalate intert-butanol was prepared in the same manner as in Example 1 except that4.5 g of tert-butanol was used instead of 1-butanol for 3.0 g (9.9 mmol)of silver oxalate. Then, in the same manner as in Example 1, the aminemixture liquid was dropped into the obtained slurry of silver oxalate intert-butanol to form a white substance (silver oxalate-amine complex).

After the silver oxalate-amine complex was formed, the reaction mixturewas heated with stirring and, as a result, was brought into a refluxstate at 96° C., and the temperature of the reaction mixture did notrise any further. The silver oxalate-amine complex was thermallydecomposed at this reflux temperature of 96° C. to obtain a suspensionin which deep blue silver nano-particles were suspended in the aminemixture liquid. In both the complex-forming reaction step and thethermal decomposition step, stirring could be very successfullyperformed.

Then, in the same manner as in Example 1, a silver nano-particledispersion liquid was prepared using the obtained suspension, and acalcined silver film having a thickness of about 0.7 μm was formed. Thespecific resistance value of the obtained calcined silver film wasmeasured by a four-terminal method and found to be 12.9 μΩcm.

Comparative Example 1

In preparation of silver nano-particles, a slurry of silver oxalate inmethanol was prepared in the same manner as in Example 1 except that 4.5g of methanol was used instead of 1-butanol for 3.0 g (9.9 mmol) ofsilver oxalate. Then, in the same manner as in Example 1, the aminemixture liquid was dropped into the obtained slurry of silver oxalate inmethanol to form a white substance (silver oxalate-amine complex).

After the silver oxalate-amine complex was formed, the reaction mixturewas heated with stirring and, as a result, was brought into a refluxstate at 86° C., and the temperature of the reaction mixture did notrise any further. The silver oxalate-amine complex was thermallydecomposed at this reflux temperature of 86° C. to obtain a suspensionin which deep blue silver nano-particles were suspended in the aminemixture liquid.

Then, the obtained suspension was treated in the same manner as inExample 1, but the silver nano-particles were not dispersed in a mixedsolvent of 1-hexanol/decalin (weight ratio=1/1) (namely, the silvernano-particles did not pass through a 0.45-μm filter), and therefore acoating film could not be formed.

The invention claimed is:
 1. A method for producing silvernano-particles comprising: mixing an aliphatic hydrocarbon amine and asilver compound in the presence of an alcohol solvent having 3 or morecarbon atoms to form a complex compound comprising the silver compoundand the amine; and thermally decomposing the complex compound by heatingto form silver nano-particles, wherein the aliphatic hydrocarbon aminecomprises an aliphatic hydrocarbon monoamine (A) comprising an aliphatichydrocarbon group and one amino group, said aliphatic hydrocarbon grouphaving 6 or more carbon atoms in total, and an aliphatic hydrocarbonmonoamine (B) comprising an aliphatic hydrocarbon group and one aminogroup, said aliphatic hydrocarbon group having 2 or more and 5 or lesscarbon atoms in total, wherein the aliphatic hydrocarbon monoamine (B)is an alkylmonoamine.
 2. The method for producing silver nano-particlesaccording to claim 1, wherein the silver compound is silver oxalate. 3.The method for producing silver nano-particles according to claim 1,wherein the aliphatic hydrocarbon amine further comprises an aliphatichydrocarbon diamine (C) comprising an aliphatic hydrocarbon group andtwo amino groups, said aliphatic hydrocarbon group having 8 or lesscarbon atoms in total.
 4. The method for producing silver nano-particlesaccording to claim 3, wherein the aliphatic hydrocarbon diamine (C) isan alkylenediamine in which one of the two amino groups is a primaryamino group, and the other is a tertiary amino group.
 5. The method forproducing silver nano-particles according to claim 1, wherein thealiphatic hydrocarbon monoamine (A) is an alkylmonoamine having 6 ormore and 12 or less carbon atoms.
 6. The method for producing silvernano-particles according to claim 1, wherein the alcohol solvent isselected from the group consisting of butanols and hexanols.
 7. Themethod for producing silver nano-particles according to claim 1, whereinthe alcohol solvent is used in an amount of 120 parts by weight or moreper 100 parts by weight of the silver compound.
 8. The method forproducing silver nano-particles according to claim 1, wherein thealiphatic hydrocarbon amine is used in a total amount of 1 to 50 molesper 1 mole of silver atoms in the silver compound.